The present invention relates to a radio communication system, base station and random access channel transmission method thereof, and more particularly, to a radio communication system, base station and random access channel transmission method thereof in which a user terminal selects a preamble pattern from among a plurality of known preamble patterns and transmits that preamble pattern to the base station.
In a cellular system such as a 3GPP system, a user terminal performs a cell search immediately after the power is turned ON or immediately after a handover, and after that cell search is completed, uses a random access channel RACH to perform initial communication with a base station. The base station periodically uses a broadcast channel to broadcast a plurality of preamble patterns for initial communication used in a cell, so the user selects an appropriate preamble pattern from among the plurality of preamble patterns included in the received broadcast information, and transmits that preamble pattern to the base station. In this case, the number of preamble patterns is set so that the probability that the same preamble will be transmitted at the same time from a plurality of users is sufficiently small. The base station performs correlation processing between received signals and all of the preamble patterns that could be transmitted, determines whether or not a preamble pattern has been transmitted by whether or not a correlation peak that is equal to or greater than a set value has been detected, and sends a response. The user terminal repeatedly performs transmission a set number of times until the preamble reaches and is received by the base station, and when it is confirmed that the preamble has arrived at the base station, the user terminal transmits a message such as a terminal number, data type, data amount or the like to the base station and establishes a communication link with the base station. When the preamble does not arrive at the base station even after being transmitted the set number of times, the user terminal terminates transmission of the preamble.
The probability that a preamble will be detected by the base station depends on the reception power and the preamble length. In the next generation 3GPP system (3GPP LTE), a user terminal estimates the propagation loss of a downlink signal from the base station and determines the transmission power for first transmission of the preamble according to the amount of that loss. As a result, the reception power at the base station of a signal from the edge of a cell becomes nearly the same as that of a signal from the center of the cell, and the detection probability is nearly the same.
However, a 3GPP LTE system is capable of handling cell sizes having a radius up to at least 100 km, and there is a large difference between the maximum propagation loss of a small cell and large cell. On the other hand, the maximum transmission power is the same for all user terminals regardless of whether the cell is a small cell or large cell. Therefore, a problem occurs in that for preambles having the same preamble length, the detection probability differs depending on the size of the cell.
To solve this problem, a method of changing the length of a preamble according to the size of the cell has been proposed (see TSG-RAN WG1 #45, R1-061367). More specifically, the length of a preamble is changed by changing the number of times ‘n’ that a unit sequence C(x) of a preamble is repeated based on the cell size as shown in FIG. 17. At the same time, the size of 1 RACH slot is also changed to a unit access slot×(n+1). TG1 is the Guard Time that is provided in a RACH so that the preamble does not receive any effects from the previous channel, and TG2 is a Guard Time that is provided in a RACH so that there is no effect on the next channel. During reception, in order to make it easier to perform the necessary correlation computation using frequency domain processing, it is possible to attach a Cyclic Prefix instead of providing the guard time TG1 (see TG-RAN WG1 LTE Ad-Hoc, R1-061870). This Cyclic Prefix itself is a repetition of part of the unit sequence.
Also, in a 3GPP LTE system, use of a CAZAC (Constant Amplitude Zero Auto-Correlation) sequence as the unit sequence of a preamble pattern has been considered. As the name implies, a CAZAC sequence has an ideal auto correlation property, so a pair of sequences having the same sequence number but different cyclic shift intervals are orthogonal to each other. A preamble pattern that uses a CAZAC sequence can be defined as P(k, s, r), which is a function of the sequence number k, cyclic shift interval s and number of repetitions r. In order to be able to use a CAZAC sequence having the same sequence number as the preamble patterns for different users by changing the cyclic shift interval, it is essential that the cyclic shift interval s is greater than the difference in arrival times at the base station of the preambles transmitted from each user. In the case that the number of preamble patterns that are possible by cyclic shifting is not enough, sequences with a different sequence number are used.
A Zadoff-Chu sequence, which is a typical CAZAC sequence, is expressed by Equation (1) (see B. M. Popovic, “Generalized Chirp-Like Polyphase Sequences with Optimum Correlation Properties”, IEEE Trans. Infor. Theory, Vol. 38, pp. 1406-1409, July 1992).ZCk(n)=exp{−j2τk/L·(qn+n(n+L%2)/2}  (1)
Here, L and k are both prime values and represent the sequence length and sequence number, respectively. Also, n is the symbol number (0, 1, . . . , L−1), q is an arbitrary integer, and L %2 is the remainder after dividing L by 2 and may be also be written as Lmod(2). When L is a prime number, the number of CAZAC sequences M becomes (L−1). Therefore, when L=149, the number of CAZAC sequences M is 148, and when L=73, M=72, and when L=37, M=36. The sequence that is obtained by cyclically shifting ZCk(n) by just s can be expressed as ZCk(n+s). For CAZAC sequences, the Peak to Average Power Ratio PAPR characteristic changes greatly according to the sequence number. In (A) of FIG. 18, a Raw Cubic Metric (an evaluation index nearly equivalent to PAPR) for L=149, 73, 37 is shown, and (B) of FIG. 18 shows the Raw Cubic Metric for the case in which L=37. In the figure, the Raw Cubic Metrics for the data modulation methods (BPSK, QPSK and 16QAM) are shown for a comparison. Also, in (B) of FIG. 18, the Raw Cubic Metrics are shown in order of the smallest with sequence numbers assigned starting from No. 1.
In TSG-RAN WG1 #45, R1-061367 and TSG-RAN WG1 LTE Ad-Hoc, R1-061870 (R1-061367, R1-061870), the preamble pattern for a cell is designed based on the number of times it is necessary to repeat transmission from a user at the edge of a cell and the cyclic shift interval. Therefore, the preamble patterns that are used by user terminals at the edge of a cell and in the center of a cell are the same, and only the transmission power changes according to the location of the user terminal. However, in this method for designing a preamble pattern, the following problems occur.
In other words, the number of repetitions of the preamble pattern is greater for a large cell than a small cell, so the overall size of the preamble becomes longer, and a problem occurs in that the processing load become large on the receiving side (base station). Also, it is necessary to set the cyclic shift interval s so that it is longer than the maximum delay time from the users at the edge of the cell. In order to accomplish this, the cyclic shift interval must be made to be large in large cells, however a problem occurs in that the number of sequences (number of preamble patterns) that can be taken through cyclic shifting from a primary sequence having little mutual interference decreases, thus another primary sequence having large mutual interference must be used.